1. Field of the Invention
The present invention relates to a light guide, a light source device equipped with the light guide and a display device using this device and an information terminal, and more in particular, it relates to a light source device having a light guide which is uniform in the intensity of distribution of an emitting light and high in emitting efficiency, and a display device which is high in cognizability and is in need of little electric power consumption and an information terminal equipped therewith.
2. Description of the Prior Art
Among the display devices, a liquid crystal display (LCD) is characterized by being light in weight and thin in size. Consequently, the LCD has been widely used as the display device for use of information processing terminals of a laptop type, a note type and the like or as the display device of various information terminals such as mobile electronic equipment including a portable type television, an electronic notebook, a portable telephone and the like. In general, in the LCD, the cognizability of information is enhanced by illuminating a display panel by a light source device.
As one of the light source devices used in the LCD, there is available an edge light type light source device, in which a light from the light source is allowed to enter from a side surface of a sheet-shaped light guide which becomes an emitting surface. Since the edge light type light source device is placed at the side surface of an almost bar-shaped light guide, the thickness of the whole light source device can be made nearly equal to the light guide. Consequently, it is possible for the edge light type light source device to be made thinner compared to a vertical type light source device having a light source placed in the front surface of the sheet-shaped light guide. Thus, in the LCD, use of the edge light type light source device has been prevailing.
As for the light source adopted for the edge light type light source device, there is available a point light source such as a light emitting diode (LED) and the like or a line light source such as a cold cathode fluorescent lamp (CCFL).
In FIG. 12 is shown a constitution of the light source device, in which a light from the point light source is turned into a line light source (turned into a linear light flux), and after that, it is allowed to enter the sheet-shaped light guide. The light source device 101 is constituted by the point light sources 102a, 102b, a linear light guide 103, and the sheet-shaped light guide 104. In a direction to which the point light sources 102a and 102b emit a light, there is provided the linear light guide 103, and in a direction to which the linear light guide 103 emits a light, there is provided the sheet-shaped light guide 104.
In the linear light guide 103, there is formed a reflecting portion 105 provided with a periodic concave-convex structure or a light scattering portion on a surface opposed to an emitting surface 103c. Further, in a sheet-shaped light guide 104, there is formed a reflecting portion 106 comprising the periodic concave-convex structure or the light scattering portion on the surface opposed to an emitting surface 104c. 
The light which emits from the point light sources 102a and 102b and enters inside the linear light guide 103 from incidence planes 103aand 103b is reflected on a reflecting portion 105, so that it is turned into a line light source and is emitted outside of the linear light guide 103 from the emitting surface 103c. The light which emits from the linear light guide 103 and enters inside the sheet-shaped light guide 104 from an incidence plane 104a is reflected on a reflecting portion 106, so that it is turned into a sheet light source (turned into a sheet-shaped light flux) with its angle changed and is emitted outside of the sheet-shaped light guide 104 from an emitting surface 104c. 
Although not shown in the drawing, the display device is disposed in such a way as to oppose to the emitting surface 104c of the sheet-shaped light guide 104. Incidentally, the light source device 101 is used as a back light in case of illuminating a transmission type display device, and as a front light in case of illuminating a reflection type display device. FIG. 12 shows a constitution adopting the light source device 101 as a front light.
Further, in FIG. 13 is shown a constitution of the light source device of a type which allows the light emitted by the line light source (such as CCFL and the like) to directly enter the sheet-shaped light guide. The light source device 101 is constituted by line light sources 108a and 108b, a sheet-shaped light guide 104,and reflectors 109a and 109b. Line light sources 108a and 108b are disposed by opposing to incidence planes 104a and 104b of the sheet-shaped light guide 104. The incidence planes 104a and 104b are covered by the reflectors 109a and 109b together with the line light sources 108a and 108b. 
Incidentally, in the sheet-shaped light guide 104, there is formed a reflecting portion 106 comprising the periodic concave-convex structure or the light scattering portion on the surface opposed to the emitting surface 104c. 
The light emitted from the line light sources 108a and 108b is reflected directly or in the reflectors 109a and 109b, and after that, it enters the sheet-shaped light guide 104 and is reflected in the reflecting portion 106, so that it is turned into a sheet light source.
Although not shown in the drawing, the display device is disposed in such a way as to oppose to the emitting surface 104c of the sheet-shaped light guide 104. Incidentally, FIG. 13 shows a constitution adopting the light source device 101 as a back light.
In recent years, demands for the enhancement of cognizability and reduction in electric power consumption for the LCD have been increased.
In the above-described edge light type light source device, the light entering the linear light guide or the sheet-shaped light guide from the light source repeats a reflection inside the light guide during the course of turning into a line light source or a sheet light source. At this time, the light is not only absorbed in the light guide and the light scattering portion, but the part thereof returns to the light source and the reflector so as to be absorbed there, and this causes an optical loss to reduce the emitting efficiency of the light guide. As a result, there is a problem in that the optical utilization efficiency of the light source device is lowered. Incidentally, what is meant by “the emitting efficiency of the light guide” is a ratio of the light which emits outside of the light guide from the emitting surface, to the light incident on the light guide from the incidence plane. Further, what is meant by “the optical utilization efficiency of the light source device” is a ratio of the light which illuminates the display device, to the light which is emitted from the light source.
To solve this problem, there is disclosed a method for enhancing the optical utilization efficiency of the light source device by modifying a shape of the linear light guide in Japanese Patent Laid-Open No. 2002-365439.
In FIG. 14 is shown a constitution of the invention disclosed in this publication. The light source device 101 is constituted by the point light sources 102a and 102b, the linear light guide 103, and the sheet light source 104. FIG. 14A is a top view and a front view schematically representing the light source device 101, and FIG. 14B is an enlarged view of the reflecting portion 105 of the linear light guide 103.
As shown in FIG. 14A, in a direction to which the light emits from the point light sources 102a and 102b, there is disposed the linear light guide 103, and in a direction to which the light emits from the linear light guide 103, there is disposed the sheet-shaped light guide 104.
The linear light guide 103, as shown in FIG. 14A, is narrowest in its width at the center, and is formed so as to become wide toward both end portions. In the linear light guide 103, there is formed the reflecting portion 105 on the surface opposed to the emitting surface 103c. 
The reflecting portion 105, as shown in FIG. 14B, is constituted by a total reflecting surface 105a, which guides the light incident on the linear light guide 103 from the point light sources 102a and 102b (in other words, guides the light without causing an optical loss) and a light extracting surface 105b, which reflects the light advancing inside the linear light guide 103 in such a way that it emits from the emitting surface 103c. The light extracting surface 105b forms an almost V-shaped groove, and allows the light incident from both of the point light sources 102a and 102b to reflect. The total reflecting surface 105a and the light extracting surface 105b are formed in such a way as to gradually draw to the emitting surface 103c as it draws to the center from the end of the linear light guide 103. The linear light guide 103 is narrowest in its width at the center of the emitting surface 103c, and is formed in such a way as to become wide toward both end portions.
In the sheet-shaped light guide 104, there is provided the reflecting portion 106 comprising the periodic concave-convex portion or the light scattering portion on a surface opposed to the emitting surface 104c. 
The light incident on the linear light guide 103 from the point light sources 102a and 102b is reflected on the reflecting portion 105, and after being turned into a line light source, it emits from the linear light guide 103, and enters the sheet-shaped light guide 104 from the incidence plane 104a of the sheet-shaped light guide 104. The light incident on the sheet-shaped light guide 104 is reflected on the reflecting portion 106 so as to be turned into a sheet light source, and emits from the emitting surface 104c. 
As described above, the linear light guide 103 is narrowest in its width at the center and is wide toward both end portions. Consequently, the majority of the light incident on the linear light guide 103 from the point light sources 102a and 102b emits outside of the linear light guide 103 before reaching the center in which the width of the linear light guide 103 becomes narrowest, and this causes the number of reflection times to be reduced at the linear light guide 103. As a result, the optical loss at the linear light guide 103 is lowered, thereby enhancing the optical utilization efficiency of the light source device 101.
As for another method, there is a method disclosed in Japanese Patent Laid-Open No. 2001-243822, in which a shape of the reflecting portion of the sheet-shaped light guide is improved so as to enhance emitting efficiency of the sheet-shaped light guide.
In FIG. 15 is shown a constitution of the light source device of this publication. The light source device 101 is constituted by the point light source 102, the linear light guide 103, and the sheet-shaped light guide 104. FIG. 15A is a schematic top view and a schematic front view of the light source device 101, and FIG. 15B is an enlarged view of the reflecting portion 106 of the sheet-shaped light guide 104.
As shown in FIG. 15A, in a direction to which the light emits from the point light source 102, there is disposed the linear light guide 103, and in a direction to which the light emits from the linear light guide 103, there is disposed the sheet-shaped light guide 104.
Here, the linear light guide 103 is wedge-shaped. Further, in the sheet-shaped light guide 104, there is formed the reflecting portion 106 on the surface opposed to the emitting surface 104c. The reflecting portion 106, as shown in FIG. 15B, is constituted by the total reflecting surface 106a which guides the light incident on the sheet-shaped light guide 104, the light reflecting surface 106b which allows the light (advancing inside the light guide without causing a loss) which is guided inside the sheet-shaped light guide 104 to reflect in such a way as to emit from the emitting surface 104c, and a re-incidence plane 106c which takes the light transmitting the light extracting surface 106b into the sheet-shaped light guide 104 again. A tilt angle p of the light extracting surface 106b and a tilt angle q of the re-incidence plane 106c are in the relationship of p<q, p=45 to 65°, and q=80 to 90°.
The light emitted from the point light source 102 and incident on the linear light guide 103 is incident on the sheet-shaped light guide 104 after it is turned into a line light source by the linear light guide 103, and it is reflected on the reflecting portion 106, so that it is turned into a sheet light source.
In the light source device 101, a part of the light transmitted outside of the sheet-shaped light guide 104 from the light extracting surface 106b is incident again inside the sheet-shaped light guide 104 from the re-incidence plane 106c immediately after transmitting, and emits from the emitting surface 104 after it is reflected on the adjacent light extracting surface 106b. As a result, a number of reflection times at the sheet-shaped light guide 104 is reduced, and the optical loss at the sheet-shaped light guide 104 is lowered, thereby enhancing the optical utilization efficiency of the light source device 101.
Although the light source device 101 shown in FIG. 14 attempts to enhance the emitting efficiency by reducing the number of reflection times in the linear light guide 103, the intensity distribution of an emitted light from the linear light guide 103 is not made uniform because the number of reflection times is reduced. That is, as shown in FIG. 16A, the emitted light from the linear light guide 103 shows the intensity distribution in which it is strong in the vicinity of both end portions (incidence planes 103a and 103b) of the linear light guide 103 and becomes weak as it draws to the center portion. As a result, the uniformity of the intensity distribution of the emitted light from the light source device 101 becomes low, and the cognizability of the display device using the light source device 101 is also lowered.
Further, the same is applicable to the light source device 101 shown in FIG. 15. As shown in FIG. 16B, the emitted light from the sheet-shaped light guide 104 shows the intensity distribution in which it is strong in the vicinity of the end portion (incidence plane 104a) of the sheet-shaped light guide 104 and becomes weak as it is apart from the incidence plane 104a. 
On the other hand, in Japanese Patent Laid-Open Nos. 2001-332112 and 2002-40420, there is disclosed an invention aiming at the uniformity of the intensity distribution of the emitted light from the light guide.
In FIG. 17 is shown a constitution of the invention disclosed in these patent publications. A light source device 200 has a depth of a wedge-shaped reflection groove deepened as the groove is apart from the light source in order to enhance the uniformity of the intensity of the emitted light from the light guide. In this way, in the center portion of the light guide, an amount of light incident on the wedge-shaped reflection groove is increased, so that the intensity distribution of the light emitted from the light guide is made uniform.
However, in the case of such constitution, since the depth of the groove is limited by a groove pitch and a groove angle, among the light incident on the light guide, an almost-parallel light is unable to reflect effectively on the groove. As a result, the optical loss arising from the light being repeatedly reflected inside the light guide or the light being emitted from the incidence plane opposite to the incidence plane where it is incident becomes large so that the optical utilization efficiency of the light source device is lowered. That is, an amount of emitted light from the light guide is uniformed in a low state of utilization efficiency, and cognizability of the display device using this light guide becomes low.
In this way, it has been impossible for the light guide equipped with a conventional light source device to allow both the uniformity of the intensity distribution of the emitted light and the enhancement of emitting efficiency to be compatible.
The present invention has been made in view of the above-described problems, and its object is to provide a light guide which is high in emitting efficiency and uniform in the intensity distribution of the emitted light, a light source device equipped with this guide, a display device adopting this device, and an information terminal.